Introduction: Hypermobility syndrome (HMS) is a common cause of morbidity in children, with the knee most frequently affected by its symptoms. Impaired joint proprioception has been reported in adults with HMS. Muscle weakness, problems with school activities and abnormal gait patterns have been observed in children with this condition. It has also been suggested that activities of daily living and physical and sporting activities may be limited in children with HMS due to pain. To date, the factors associated with HMS in children have not been well reported. The relationships between impairments, function and quality of life (QoL) have not been investigated in children with this condition. The purpose of this study was to identify the range of neuromuscular performance, functional range of motion (ROM) and QoL indices, and investigate the relationships between these features in children with HMS. A purpose-built motorised device was developed and validated for the assessment of knee joint proprioception as an integral part of the research programme. The test-retest repeatability of various outcome measures used for the present study was also investigated in healthy children and those with HMS. Methods: A cross-sectional study was conducted. Twenty nine children with HMS and 37 healthy children (aged 8 – 15 years) were investigated for neuromuscular indices, functional ROM and QoL. Knee joint kinaesthesia (JK) and position sense (JPS) were examined using a motorised device, muscle torque was tested with a digital myometer, passive ROM was measured with a universal goniometer and functional ROM was assessed using the VICON camera system. Pain intensity and QoL were measured using the Coloured Analogue Scale and the Paediatric Quality of life Inventory respectively. Mann-Whitney U tests and independent t-tests were performed to determine the differences between the two groups. The relationships between pain and each of the following: neuromuscular impairments, functional ROM and QoL were examined in children with HMS. The correlation between Beighton scores and each outcome was also evaluated in children with HMS. Results: Knee JK and JPS were significantly poorer (both p < 0.001) in children with HMS compared with the controls. Significantly reduced (p < 0.001) knee muscle torque was also observed in children with HMS. Pain intensity and passive knee ROM were significantly higher (both p < 0.001) in children with HMS. They also demonstrated significantly increased knee extension, reduced knee flexion in loading response and during maximal knee flexion of walking (all p <0.001). Moreover, the overall QoL perception and all the domains were significantly poorer (p range < 0.001 to 0.008) in children with HMS than the controls. No relationship (r range = -0.065 to 0.271; p range = 0.106 to 0.985) was found between pain, neuromuscular impairments and functional ROM in children with HMS. However, a significantly strong negative relationship (r = -0.65; p = <0.001) was established between pain and QoL in children with HMS. In addition, no relationship (r range = -0.014 to 0.315; p range = 0.112 to 0.895) was observed between Beighton scores and neuromuscular impairments, functional ROM and QoL in children with HMS. Conclusions: Children with HMS, compared with their healthy counterparts had knee joint proprioception and knee muscle torque deficits, increased passive knee ROM and pain intensity. Abnormal walking patterns (increased knee extension, reduced knee flexion in both mid stance and maximum knee flexion in swing phase during walking) were also found in children with HMS. They also presented with poorer QoL in comparison with the controls. Clinicians are to be aware of these identified features and should develop appropriate treatment intervention programmes for children with this condition.